Lubricating oil composition

ABSTRACT

Disclosed is a lubricating oil composition comprising: a lubricating base oil comprising 0.5% to 70% by mass of an ester base oil based on the total amount of the lubricating base oil, and having a kinematic viscosity at 40° C. of 18 to 28 mm 2 /s; and an organic molybdenum compound in an amount of 100 to 1000 mass ppm in terms of the molybdenum element based on the total amount of the lubricating oil composition, wherein the lubricating oil composition has a kinematic viscosity at 40° C. of 50 mm 2 /s or less.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition.

BACKGROUND ART

In recent years, in order to cope with environmental problems such asreduction in carbon dioxide emission, the saving of energy inautomobiles, construction machineries, agricultural machineries, etc.,namely, fuel saving has become an urgent task, and thus, it has beenstrongly desired that devices such as engines, transmissions, finaldrive gears, compressors or hydraulic systems contribute to such savingof energy. As such, lubricating oil used in these devices has beenrequired to reduce stirring resistance and rotational resistance, incomparison to the conventional oil.

As a means for fuel saving in transmissions and final drive gears,reduction in the viscosity of lubricating oil is applied. For example,among transmissions, automatic transmissions and continuously variabletransmissions for use in automobiles have a torque converter, a wetclutch, a gear bearing mechanism, an oil pump, a hydraulic controlmechanism and the like, and also, manual transmissions and final drivegears have a gear bearing mechanism, and by reducing the viscosity oflubricating oil used in these devices, the stirring resistance androtational resistance of such a torque converter, a wet clutch, a gearbearing mechanism, an oil pump and the like can be reduced, and thepower transmission efficiency can be improved, so that the improvementof fuel consumption in automobiles can be achieved.

However, in order to reduce the viscosity of lubricating oil and toachieve a high viscosity index, if the viscosity of base oil isdecreased and a large amount of viscosity index improver is blended intothe oil, a reduction in the oil film thickness, which is competingperformance, causes a reduction in extreme pressure properties and wearresistance, and as a result, seizure and the like may occur, so thatdefects and the like would occur in transmissions, etc. Moreover, if theamounts of a sulfur extreme pressure agent and a phosphorus-sulfurextreme pressure agent are increased to improve extreme pressureproperties, oxidation stability would be significantly deteriorated.

As conventional lubricating oil compositions, lubricating oilcompositions produced by blending various types of additives to mineraloil- and/or synthetic oil-based lubricating base oil, which have bothfuel saving and the sufficient durability of gears, bearings, etc. havebeen proposed (see, for example, Patent Literatures 1 and 2). However,regarding fuel saving, there has been still room for improvement even inthese conventional lubricating oil compositions.

CITATION LIST Patent Literature

Patent Literature 1: JP 2008-208212 A

Patent Literature 2: JP 2009-249496 A

SUMMARY OF INVENTION Technical Problem

The present invention has been made under the aforementionedcircumstances, and it is an object of the present invention to provide alubricating oil composition, which has extreme pressure properties andwear resistance enough to achieve fuel saving, and reduces ametal-to-metal friction coefficient.

Solution to Problem

To solve the aforementioned object, the present invention provides alubricating oil composition according to [1] to [4] below, use of thecomposition according to [5] below, and use of the composition for theproduction according to [6] below.

[1] A lubricating oil composition comprising: a lubricating base oilcomprising 0.5% to 70% by mass of an ester base oil based on the totalamount of the lubricating base oil, and having a kinematic viscosity at40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in anamount of 100 to 1000 mass ppm in terms of molybdenum element based onthe total amount of the lubricating oil composition, wherein thelubricating oil composition has a kinematic viscosity at 40° C. of 50mm²/s or less.

[2] The lubricating oil composition according to [1] above furthercomprising 2% by mass or more of a copolymer consisting of an α-olefinand an ester monomer having a polymerizable unsaturated bond, whereinthe weight-average molecular weight of the copolymer is 2000 to 20000.

[3] The lubricating oil composition according to [1] or [2] abovefurther comprising a boron-containing dispersant in an amount of 100 to500 mass ppm in terms of boron element based on the total amount of thelubricating oil composition.

[4] The lubricating oil composition according to any one of [1] to [3]above, which is used for a hypoid gear.

[5] Use of the composition as a lubricating oil for a hypoid gear,wherein the composition comprises: a lubricating base oil comprising0.5% to 70% by mass of an ester base oil based on the total amount ofthe lubricating base oil, and having a kinematic viscosity at 40° C. of18 to 28 mm²/s; and an organic molybdenum compound in an amount of 100to 1000 mass ppm in terms of the molybdenum element based on the totalamount of the lubricating oil composition, wherein the lubricating oilcomposition has a kinematic viscosity at 40° C. of 50 mm²/s or less.

[6] Use of the composition for the production of a lubricating oil for ahypoid gear, wherein the composition comprises: a lubricating base oilcomprising 0.5% to 70% by mass of an ester base oil based on the totalamount of the lubricating base oil, and having a kinematic viscosity at40° C. of 18 to 28 mm²/s; and an organic molybdenum compound in anamount of 100 to 1000 mass ppm in terms of the molybdenum element basedon the total amount of the lubricating oil composition, wherein thelubricating oil composition has a kinematic viscosity at 40° C. of 50mm²/s or less.

The term “kinematic viscosity” used in the present invention means akinematic viscosity defined according to ASTM D-445. In addition, theterm “viscosity index” used in the present invention means a viscosityindex measured in accordance with JIS K 2283-1993.

Advantageous Effects of Invention

According to the present invention, there is provided a lubricating oilcomposition, which has sufficient extreme pressure properties and wearresistance, and further reduces a metal-to-metal friction coefficient.Therefore, when the lubricating oil composition of the present inventionis applied to an automotive manual transmission, an automatictransmission or a continuously variable transmission, or to anindustrial gear system, it can achieve fuel saving, while maintainingproperties necessary as a gear oil, and particularly, as a hypoid gearoil.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed.

The lubricating oil composition according to the present embodimentcomprises: (A) a lubricating base oil comprising 0.5% to 70% by mass ofan ester base oil based on the total amount of the lubricating base oil,and having a kinematic viscosity at 40° C. of 18 to 28 mm²/s; and (B) anorganic molybdenum compound in an amount of 100 to 1000 mass ppm interms of the molybdenum element based on the total amount of thelubricating oil composition, wherein the lubricating oil composition hasa kinematic viscosity at 40° C. of 50 mm²/s or less.

Component (A): Lubricating Base Oil

The lubricating oil composition of the present embodiment comprises (A)a lubricating base oil comprising 0.5% to 70% by mass of an ester baseoil based on the total amount of the lubricating base oil, and having akinematic viscosity at 40° C. of 18 to 28 mm²/s.

The alcohol constituting the ester base oil may be either monohydricalcohol or polyhydric alcohol (polyol), and the acid constituting theester base oil may be either monobasic acid or polybasic acid. Inaddition, as long as the ester base oil is a base oil containing anester bond, it may also be a complex ester compound.

As monohydric alcohols, the monohydric alcohol having generally 1 to 24,preferably 1 to 12, and more preferably 1 to 8 carbon atoms is used, andsuch alcohol may be linear or branched, and may also be saturated orunsaturated. Specific examples of the alcohol having 1 to 24 carbonatoms include methanol, ethanol, linear or branched propanol, linear orbranched butanol, linear or branched pentanol, linear or branchedhexanol, linear or branched heptanol, linear or branched octanol, linearor branched nonanol, linear or branched dccanol, linear or branchedundecanol, linear or branched dodecanol, linear or branched tridecanol,linear or branched tetradecanol, linear or branched pentadecanol, linearor branched hexadecanol, linear or blanched heptadecanol, linear orbranched octadecanol, linear or branched nonadecanol, linear or branchedicosanol, linear or branched henicosanol, linear or branched tricosanol,linear or branched tetracosanol, and the mixtures thereof.

As polyhydric alcohols (polyol), generally a 2- to 10-valent, andpreferably a 2- to 6-valent polyhydric alcohol is used. Specificexamples of the 2- to 10-valent polyhydric alcohol include: divalentalcohols such as ethylene glycol, diethylene glycol, polyethylene glycol(trimer to 15-mer of ethylene glycol), propylene glycol, dipropyleneglycol, polypropylene glycol (trimer to 15-mer of propylene glycol),1,3-propanediol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol,2-methyl-1-1,2-propanediol, 2-methyl-1,3-propanediol, 1,2-pentanediol,1,3-pentanediol, 1,4-pentanediol, 1,5-pentanediol, or neopentyl glycol;polyhydric alcohols such as glycerin, polyglycerin (dimer to octamer ofglycerin, such as diglycerin, triglycerin, or tetraglycerin),trimethylolalkane (trimethylolethane, trimethylolpropane,trimethylolbutane, etc.) and the dimer to octamer thereof,pentaerythritol and the dimer to tetramer thereof, 1,2,4-butanetriol,1,3,5-pentanetriol, 1,2,6-hexanetriol, 1,2,3,4-butanetetrol, sorbitol,sorbitan, a sorbitol glycerin condensate, adonitol, arabitol, xylitol,or mannitol; sugars such as xylose, arabinose, ribose, rhamnose,glucose, fructose, galactose, mannose, sorbose, cellobiose, maltose,isomaltose, trehalose, or sucrose; and the mixtures thereof.

As monobasic acids, a fatty acid having 2 to 24 carbon atoms isgenerally used, and the tatty acid may be linear or branched, and mayalso be saturated or unsaturated. Specific examples include: saturatedfatty acids such as acetic acid, propionic acid, linear or branchedbutanoic acid, linear or branched pentanoic acid, linear or branchedhexanoic acid, linear or branched heptanoic acid, linear or branchedoctanoic acid, linear or branched nonanoic acid, linear or brancheddecanoic acid, linear or branched undecanoic acid, linear or brancheddodecanoic acid, linear or branched tridecanoic acid, linear or branchedtetradecanoic acid, linear or branched pentadecanoic acid, linear orbranched hexadecanoic acid, linear or branched heptadecanoic acid,linear or branched octadecanoic acid, linear or branched nonadecanoicacid, linear or branched icosanoic acid, linear or branched henicosanoicacid, linear or branched docosanoic acid, linear or branched tricosanoicacid, or linear or branched tetracosanoic acid; unsaturated fatty acidssuch as acrylic acid, linear or branched butenoic acid, linear orbranched pentenoic acid, linear or branched hexenoic acid, linear orbranched heptenoic acid, linear or branched octenoic acid, linear orbranched nonenic acid, linear or branched decenoic acid, linear orbranched undecenoic acid, linear or branched dodecenoic acid, linear orbranched tridecenoic acid, linear or branched tetradecenoic acid, linearor branched pentadecenoic acid, linear or branched hexadecenoic acid,linear or branched heptadecenoic acid, linear or branched octadecenoicacid, linear or branched nonadecenoic acid, linear or branched icosenoicacid, linear or branched henicosenoic acid, linear or brancheddocosenoic acid, linear or branched tricosenoic acid, or linear orbranched tetracosenoic acid, and the mixtures thereof.

Examples of the polybasic acid include dibasic acid having 2 to 16carbon atoms and trimellitic acid. The dibasic acid having 2 to 16carbon atoms may be linear or branched, and may also be saturated orunsaturated. Specific examples include ethanedioic acid, propanedioicacid, linear or branched butanedioic acid, linear or branchedpentanedioic acid, linear or branched hexanedioic acid, linear orbranched heptanedioic acid, linear or branched octanedioic acid, linearor branched nonanedioic acid, linear or branched decanedioic acid,linear or branched undecanedioic acid, linear or branched dodecanedioicacid, linear or branched tridecanedioic acid, linear or branchedtetradecanedioic acid, linear or branched heptadecanedioic acid, linearor branched hexadecanedioic acid, linear or branched hexenedioic acid,linear or branched heptenedioic acid, linear or branched octenedioicacid, linear or branched nonenedioic acid, linear or brancheddecenedioic acid, linear or branched undecenedioic acid, linear orbranched dodecenedioic acid, linear or branched tridecenedioic acid,linear or branched tetradecenedioic acid, linear or branchedheptadecenedioic acid, linear or branched hexadecenedioic acid, and themixtures thereof.

The combination of alcohol and acid that form an ester are arbitrarilyselected, and is not particularly limited, and examples of the esterthat can be used in the present invention include the following esters,and these esters may be used singly or in combinations of two or more:

(a) an ester formed from monohydric alcohol and monobasic acid,

(b) an ester formed from polyhydric alcohol and monobasic acid,

(c) an ester formed from monohydric alcohol and polybasic acid,

(d) an ester formed from polyhydric alcohol and polybasic acid,

(e) a mixed ester formed from a mixture of monohydric alcohol andpolyhydric alcohol, and polybasic acid,

(f) a mixed ester formed from polyhydric alcohol, and a mixture ofmonobasic acid and polybasic acid, and

(g) a mixed ester formed from a mixture of monohydric alcohol andpolyhydric alcohol, monobasic acid, and polybasic acid.

Among these esters, (c) the ester formed from monohydric alcohol andpolybasic acid is preferable because it is excellent in abrasionresistance and oxidation stability, and a dibasic acid ester that is anester formed from monohydric alcohol and dibasic acid is morepreferable.

The content of the ester base oil is 0.5% to 70% by mass, preferably 1%by mass or more, more preferably 2% by mass or more, and even morepreferably 3% by mass or more, based on the total amount of thelubricating base oil. Also, it is preferably 60% by mass or less, andmore preferably 55% by mass or less. If the content of the ester baseoil is 0.5% by mass or more, the lubricating base oil tends to beexcellent in extreme pressure properties, wear resistance, seizureresistance, and abrasion resistance. In addition, if the content of theester base oil is 70% by mass or less, the lubricating base oil tends tobe excellent in oxidation stability.

The kinematic viscosity of the ester base oil at 40° C. is notparticularly limited, and it is preferably 5 mm²/s or more, morepreferably 6 mm²/s or more, and even more preferably 7 mm²/s or more.Also, it is preferably 50 mm²/s or less, more preferably 30 mm²/s orless, and even more preferably 20 mm²/s or less. If the kinematicviscosity at 40° C. is 5 mm²/s or more, or 50 mm²/s or less, thelubricating base oil tends to be excellent in extreme pressureproperties, wear resistance, and seizure resistance.

The viscosity index of the ester base oil is not particularly limited,and it is preferably 125 or more, more preferably 130 or more, and evenmore preferably 135 or more. If the viscosity index is 125 or more, thelubricating base oil tends to be excellent in low temperature fluidity.

The pour point of the ester base oil is not particularly limited, and itis preferably −30° C. or lower, more preferably −50° C. or lower, evenmore preferably −60° C. or lower, and particularly preferably −70° C. orlower.

The flash point of the ester base oil is not particularly limited, andit is preferably 200° C. or higher, more preferably 250° C. or higher,and even more preferably 300° C. or higher.

The lubricating base oil according to the present embodiment maycomprise base oil components other than the ester base oil, as long asthe content of the ester base oil is 0.5% to 70% by mass based on thetotal amount of the lubricating base oil. The base oil components otherthan the ester base oil are not particularly limited, and base oil usedin ordinary lubricating oil can be used. Specific examples of the oilcomponents other than the ester base oil that can be used herein includemineral oil base oil, synthetic base oil, and a mixture obtained bymixing two or more types of base oils selected from the aforementionedbase oils at any given ratio.

Examples of the mineral oil base oil include: paraffinic and naphthenicmineral oil base oils, which are obtained by purifying a lubricating oilfraction, which has been obtained by subjecting crude oil to atmosphericdistillation and vacuum distillation, by applying purificationtreatments, such as solvent deasphalting, solvent extraction,hydrogenolysis, solvent dewaxing, catalytic dewaxing, hydrorefining,sulfuric acid washing, or clay treatment, alone or appropriately incombinations of two or more; and base oils produced by subjecting normalparaffin, isoparaffin and petroleum wax to catalytic dewaxing. It is tobe noted that these base oils may be used singly or in combinations oftwo or more at any given ratio.

From the viewpoint of reduction in viscosity and the content of sulfur,the mineral oil base oil is preferably base oil classified into Group IIor Group III in Base Stock Categories of API (American PetroleumInstitute), and more preferably base oil classified into Group III.

Examples of the synthetic base oil include a poly-α-olefin or a hydridethereof, an isobutene oligomer or a hydride thereof, isoparaffin,alkylbenzene, alkylnaphthalene, polyoxyalkylene glycol, dialkyl diphenylether, polyphenyl ether, and base oil produced by subjecting waxproduced by the Fischer-Tropsch process to catalytic dewaxing.

The synthetic base oil is preferably a poly-α-olefin, or base oilproduced by subjecting wax produced by the Fischer-Tropsch process tocatalytic dewaxing. Specific examples of the poly-α-olefin includeα-olefin oligomers or cooligomers having 2 to 32, and preferably 6 to 16carbon atoms (e.g., a 1-octene oligomer, a 1-decene oligomer, a1-dodecene oligomer, an ethylene-propylene cooligomer, etc.), and thehydrides thereof.

A method for producing a poly-α-olefin is not particularly limited, andan example of the production method is polymerization of an α-olefin inthe presence of a polymerization catalyst comprising a complex withaluminum trichloride, boron trifluoride, or boron trifluoride and water,alcohol (e.g., ethanol, propanol or butanol), carboxylic acid, or ester(e.g., ethyl acetate or ethyl propionate), such as a Friedel-Craftscatalyst.

The kinematic viscosity of the lubricating base oil at 40° C. is 18 to28 mm²/s, preferably 20 mm²/s or more, and more preferably 22 mm²/s ormore. Also, it is preferably 27 mm²/s or less, and more preferably 26mm²/s or less. By setting the kinematic viscosity at 40° C. to 18 mm²/sor more, an oil film is sufficiently formed, and thus, it becomespossible to obtain a lubricating oil composition, which is excellent inlubricity and has a smaller evaporation loss of the base oil underhigh-temperature conditions. On the other hand, by setting the kinematicviscosity at 40° C. to 28 mm²/s or less, a lubricating oil compositionbecomes excellent in low temperature fluidity and fluid resistancethereof is decreased, and thus, it becomes possible to obtain alubricating oil composition having smaller rotational resistance.

The kinematic viscosity of the lubricating base oil at 100° C. is notparticularly limited, and it is preferably 1 mm²/s or more, morepreferably 3 mm²/s or more, and even more preferably 4 mm²/s or more.Also, it is preferably 10 mm²/s or less, more preferably 8 mm²/s orless, and even more preferably 6 mm²/s or less. By setting the kinematicviscosity at 100° C. to 1 mm²/s or more, an oil film is sufficientlyformed, and thus, it becomes possible to obtain a lubricating oilcomposition, which is excellent in lubricity and has a smallerevaporation loss of the base oil under high-temperature conditions. Onthe other hand, by setting the kinematic viscosity at 100° C. to 10mm²/s or less, it becomes possible to obtain a lubricating oilcomposition, which is excellent in low temperature fluidity.

The viscosity index of the lubricating base oil is not particularlylimited, and it is preferably 120 or more, more preferably 125 or more,and even more preferably 130 or more. By setting the viscosity index to120 or more, a lubricating oil composition, which exhibits goodviscosity properties in a temperature range from a low temperature to ahigh temperature, and is excellent in oxidation stability.

Component (B): Organic Molybdenum Compound

The lubricating oil composition according to the present embodimentcomprises, as a friction modifier, an organic molybdenum compound in anamount of 100 to 1000 mass ppm in terms of the molybdenum element basedon the total amount of the lubricating oil composition. By combining thecomponent (B) with the component (A), a metal-to-metal frictioncoefficient can be reduced, and fuel saving can be enhanced.

Examples of the organic molybdenum compound according to the presentembodiment include: organic molybdenum compound containing sulfur, suchas molybdenum dithiophosphate or molybdenum dithiocarbamate (MoDTC);complexes formed from molybdenum compounds (e.g., molybdenum oxides suchas molybdenum dioxide or molybdenum trioxide, molybdic acids such asorthomolybdic acid, paramolybdic acid or polysulfurized molybdic acid,the metal salts of these molybdic acids, molybdates such as ammoniumsalt, molybdenum sulfides such as molybdenum disulfide, molybdenumtrisulfide, molybdenum pentasulfide or molybdenum polysulfide,sulfurized molybdic acids, the metal salts or amine salts of thesulfurized molybdic acids, halogenated molybdenums such as molybdenumchloride, etc.), and sulfur-containing organic compounds (e.g.,alkyl(thio)xanthate, thiaziazole, mercaptothiadiazole, thiocarbonate,tetrahydrocarbylthiuramdisufilde,bis(di(thio)hydrocarbyldithiophosphonate)disulfide, organic(poly)sulfide, and sulfide ester, etc.) or other organic compounds; andcomplexes formed from sulfur-containing molybdenum compounds such as theabove-mentioned molybdenum sulfides or sulfurized molybdenum acids andalkenyl succinimides.

Moreover, as such an organic molybdenum compound, an organic molybdenumcompound, which does not contain sulfur as a constitutional element, canbe used. Specific examples of the organic molybdenum compound, whichdoes not contain sulfur as a constitutional element, include amolybdenum-amine complex, a molybdenum-succinimide complex, themolybdenum salt of organic acid, and the molybdenum salt of alcohol, andamong others, a molybdenum-amine complex, the molybdenum salt of organicacid, and the molybdenum salt of alcohol are preferable.

In the lubricating oil composition according to the present embodiment,the content of the organic molybdenum compound is 100 to 1000 mass ppm,preferably 200 mass ppm or more, and more preferably 300 mass ppm ormore, in terms of the molybdenum element based on the total amount ofthe lubricating oil composition. Also, it is preferably 900 mass ppm orless, and more preferably 800 mass ppm or less. If the content of theorganic molybdenum compound is 100 mass ppm or more, the lubricating oilcomposition tends to be excellent in wear resistance and abrasionresistance, and if the content is 1000 mass ppm or less, the lubricatingoil composition tends to be excellent in seizure resistance. It is to benoted that the amount of the organic molybdenum compound in terms of themolybdenum element can be obtained, for example, by an ICP elementalanalysis method or the like.

The lubricating oil composition according to the present embodiment mayfurther comprise, as a viscosity modifier, a copolymer consisting of anα-olefin and an ester monomer having a polymerizable unsaturated bond,in an amount of 2% by mass or more based on the total amount of thelubricating oil composition. The weight-average molecular weight of theabove described copolymer is preferably 2000 to 20000. By allowing thepresent lubricating oil composition to further comprise such acopolymer, the oil film retentivity and extreme pressure properties ofthe lubricating oil composition can be further improved.

The ester monomer having a polymerizable unsaturated bond is notparticularly limited, as long as it is a compound having a polymerizableunsaturated bond and an ester bond, and the ester monomer is preferablyan α,β-ethylenically unsaturated dicarboxylic acid diester, which is adiester body of unsaturated dicarboxylic acid, in which the α carbon andβ carbon of at least one carboxy group form an ethylenically unsaturatedbond (namely, a C═C double bond). Herein, the α,β-ethylenicallyunsaturated dicarboxylic acid is not limited to a compound in which an αcarbon and a β carbon form an ethylenically unsaturated bond in bothcarboxy groups and the α,β-ethylenically unsaturated bond is present inthe main chain, such as maleic acid, fumaric acid, citraconic acid ormesaconic acid, but the α,β-ethylenically unsaturated dicarboxylic acidused herein means a concept including a compound in which an α carbonand a β carbon form an ethylenically unsaturated bond in only onecarboxy group, such as glutaconic acid, or a concept including acompound in which the α,β-ethylenically unsaturated bond is found in theside chain, such as itaconic acid.

The structure of the copolymer consisting of an α-olefin and an estermonomer having a polymerizable unsaturated bond is not particularlylimited, as long as its weight-average molecular weight is 2000 to20000. In addition, a method for producing the copolymer is notparticularly limited, either, and a copolymer produced by a known methodcan be used.

The weight-average molecular weight (Mw) of the copolymer consisting ofan α-olefin and an ester monomer having a polymerizable unsaturated bondis 2000 to 20000, preferably 4000 or more, and more preferably 6000 ormore. Also, the weight-average molecular weight is preferably 15000 orless, and more preferably 12000 or less. By setting the weight-averagemolecular weight at 2000 to 20000, it becomes possible to improve oilfilm retentivity and extreme pressure properties.

It is to be noted that the term “weight-average molecular weight” usedherein means a weight-average molecular weight relative to standardpolystyrene, which is measured using, in series, two columns of GMHHR-M(7.8 mm ID×30 cm) manufactured by Tosoh Corporation in the 150-C ALC/GPCdevice manufactured by Waters, using tetrahydrofuran as a solvent, andalso using a refractive index (RI) detector under conditions of atemperature of 23° C., a flow rate of 1 mL/min., a sample concentrationof 1% by mass, and a sample injection rate of 75 μL.

In the lubricating oil composition according to the present embodiment,the content of the copolymer is preferably 2% by mass or more, morepreferably 2.5% by mass or more, and even more preferably 3.5% by massor more, based on the total amount of the lubricating oil composition.By setting the content of the copolymer to 2% by mass or more, thepresent lubricating oil composition tends to be excellent in extremepressure properties and wear resistance. On the other hand, the upperlimit of the content is not particularly limited, and it is preferably25% by mass or less, more preferably 24% by mass or less, and even morepreferably 22% by mass or less. By selling the content of a component(D) to 25% by mass or less, the lubricating oil composition tends toexhibit sufficient extreme pressure properties, wear resistance, seizureresistance, abrasion resistance and oxidation stability.

The lubricating oil composition according to the present embodiment mayfurther comprise a boron-containing dispersant in an amount of 100 to500 mass ppm in terms of the boron element based on the total amount ofthe lubricating oil composition. Thereby, the oil film retentivity andextreme pressure properties of the present lubricating oil compositioncan be further improved.

The boron-containing dispersant is any given ashless dispersant that hasbeen borated. Examples of the ashless dispersant include anitrogen-containing compound having at least one linear or branchedalkyl group or alkenyl group having 40 to 400 carbon atoms in themolecule thereof, or a derivative thereof, and a modified product ofalkenyl succinimide. One or more types arbitrarily selected from theseproducts may be blended into the lubricating oil composition.

It is to be noted that the succinimide includes, what is called, amono-type succinimide represented by the following formula (3), in whichsuccinic anhydride is added to one end of polyamine, and what is called,a bis-type succinimide represented by the following formula (4), inwhich succinic anhydride is added to both ends of polyamine.

In the above formula (3), R⁹ represents an alkyl group or alkenyl grouphaving 40 to 400 carbon atoms, and preferably an alkyl group or alkenylgroup having 60 to 350 carbon atoms; and p represents an integer of 1 to5, and preferably of 2 to 4.

In the above formula (4), R¹⁰ and R¹¹ may be the same or different, andeach represent an alkyl group or alkenyl group having 40 to 400 carbonatoms, preferably an alkyl group or alkenyl group having 60 to 350carbon atoms, and preferably, each represent a polybutenyl group; and qrepresents an integer of 0 to 4, and preferably of 1 to 3.

The lubricating oil composition according to the present embodiment maycomprise either one of the mono-type and bis-type succinimides, or mayalso comprise both of them.

A method for producing a succinimide is not particularly limited, andthe succinimide can be obtained, for example, by allowing alkyl succinicacid or alkenyl succinic acid, which has been obtained by reacting acompound having an alkyl group or alkenyl group having 40 to 400 carbonatoms with maleic anhydride at a temperature of 100° C. to 200° C., toreact with polyamine. Specific examples of the polyamine includediethylenetriamine, triethylenetetramine, tetraethylenepentamine, andpentaethylenehexamine.

In the lubricating oil composition according to the present embodiment,the content of the boron-containing dispersant is preferably 100 to 500mass ppm, more preferably 150 mass ppm or more, and even more preferably200 mass ppm or more, in terms of the boron element based on the totalamount of the lubricating oil composition. Also, it is more preferably450 mass ppm or less, and even more preferably 400 mass ppm or less. Ifthe content of the boron-containing dispersant in the lubricating oilcomposition is 100 mass ppm or more, the present lubricating oilcomposition tends to be excellent in extreme pressure properties, wearresistance, seizure resistance, and abrasion resistance. Also, if thecontent of the boron-containing dispersant in the lubricating oilcomposition is 500 mass ppm or less, the present lubricating oilcomposition tends to be excellent in wear resistance. It is to be notedthat the amount of the boron-containing dispersant in terms of the boronelement can be obtained, for example, by an ICP elemental analysismethod or the like.

In order to improve performance, the lubricating oil compositionaccording to the present embodiment may further comprise any givenadditives, which are commonly used in lubricating oil, depending onpurpose. Examples of such additives include viscosity modifiers otherthan the above described copolymer, metallic detergents, ashlessdispersants other than boron-containing dispersants, wear inhibitors (orextreme pressure agents), antioxidants, corrosion inhibitors, rustinhibitors, demulsifiers, metal deactivators, defoaming agents, andfriction modifiers other than the component (B).

Specific examples of the viscosity modifier other than the abovedescribed copolymer include viscosity modifiers containing anon-dispersed or dispersed ester group, such as a non-dispersed ordispersed poly(meth)acrylate viscosity modifier, a non-dispersed ordispersed olefin-(meth)acrylate copolymer viscosity modifier, astyrene-maleic anhydride ester copolymer viscosity modifier, and themixtures thereof, and among others, a non-dispersed or dispersedpoly(meth)acrylate viscosity modifier is preferable. In particular, anon-dispersed or dispersed polymethacrylate viscosity modifier ispreferable.

Other examples of the viscosity modifier other than the above describedcopolymer include a non-dispersed or dispersed ethylene-α-olefincopolymer or a hydride thereof, polyisobutylene or a hydride thereof, astyrene-diene hydrogenated copolymer, and polyalkylstyrene.

Examples of the metallic detergent include a sulfonate detergent, asalicylate detergent and a phenate detergent, and any one of normalsalt, basic normal salt, and perbasic salt with alkaline metal oralkaline earth metal can be blended. Upon the use, one or two or moretypes, which are arbitrarily selected from these substances, can beblended into the lubricating oil composition.

As ashless dispersants other than the boron-containing dispersant, anygiven non-boron ashless dispersants, which are used in lubricating oil,can be used, and examples of such an ashless dispersant include a mono-or bis-succinimide having at least one linear or branched alkyl group oralkenyl group having 40 to 400 carbon atoms in the molecule thereof, abenzylamine having at least one alkyl group or alkenyl group having 40to 400 carbon atoms in the molecule thereof, a polyamine having at leastone alkyl group or alkenyl group having 40 to 400 carbon atoms in themolecule thereof, and the modified products thereof, which are preparedby using carboxylic acid, phosphoric acid or the like. Upon the use, oneor two or more types, which are arbitrarily selected from thesesubstances, can be blended into lubricating oil composition.

As wear inhibitors (or extreme pressure agents), any given wearinhibitors and/or extreme pressure agents, which are used in lubricatingoil, can be used. For example, sulfur-based, phosphorus-based, andsulfur-phospborus-based extreme pressure agents and the like can beused, and specific examples of such an extreme pressure agent includezinc dialkyldithiophosphate (ZnDTP), phosphite esters, thiophosphiteesters, dithiophosphite esters, trithiophosphite esters, phosphoric acidesters, thiophosphoric acid esters, dithiophosphoric acid esters,trithiophosphoric acid esters, the amine salts thereof, the metal saltsthereof, the derivatives thereof, dithiocarbamate, zinc dithiocarbamate,MoDTC, disulfides, polysulfides, sulfurized olefins, and sulfurized oilsand fats. Among these substances, addition of sulfur-based extremepressure agents is preferable, and sulfurized oils and fats areparticularly preferable.

Examples of the antioxidant include ashless antioxidants such asphenolic ashless antioxidants or amine-based ashless antioxidants, andmetallic antioxidants such as copper-based or molybdenum-basedantioxidants. Specific examples of the phenolic ashless antioxidantinclude 4,4′-methylenebis(2,6-di-tert-butylphenol) and4,4′-bis(2,6-di-tert-butylphenol; and examples of the amine-basedashless antioxidant include phenyl-α-naphthylamine,alkylphenyl-α-naphthylamine, and dialkyldiphenyl amine.

Examples of the corrosion inhibitor include benzotriazole-based,tolyltriazole-based, thiadiazole-based, and imidazole-based compounds.

Examples of the rust inhibitor include petroleum sulfonate, alkylbenzenesulfonate, dinonylnaphthalene sulfonate, alkenyl succinic acid ester,and polyhydric alcohol ester.

Examples of the demulsifier include polyalkylene glycol-based nonionicsurfactants such as polyoxyethylene alkyl ether, polyoxyethylenealkylphenyl ether, or polyoxyethylene alkylnaphthyl ether.

Examples of the metal deactivator include imidazoline, a pyrimidinederivative, alkylthiadiazole, mercaptobenzothiazole, benzotriazole or aderivative thereof, 1,3,4-thiadiazole polysulfide,1,3,4-thiadiazolyl-2,5-bisdialkyldithiocarbamate,2-(alkylthio)benzoimidazole, and β-(o-carboxybenzylthio)propionitrile.

Examples of the defoaming agent include silicone oil, an alkenylsuccinic acid derivative, an ester of polyhydroxy aliphatic alcohol anda long chain fatty acid, and an ester of methyl salicylate ando-hydroxybenzyl alcohol, of which kinematic viscosity at 25° C. is 1000to 100000 mm²/s, respectively.

Examples of the friction modifier other than the component (B) includeashless friction modifiers, and any given compounds, which are generallyused as ashless friction modifiers for lubricating oil, can be used, andexamples of such an ashless friction modifier include amine-based,imide-based, fatty acid ester-based, fatty acid amide-based, fattyacid-based, aliphatic alcohol-based, and aliphatic ether-based ashlessfriction modifiers, each of which has at least one hydrocarbon grouphaving 6 to 30 carbon atoms, preferably at least one alkyl group oralkenyl group, and particularly preferably at least one linear alkylgroup or linear alkenyl group having 6 to 30 carbon atoms, in themolecule thereof.

When these additives are contained in the lubricating oil compositionaccording to the present embodiment, the content of each additive ispreferably 0.01% to 20% by mass based on the total amount of thelubricating oil composition.

The kinematic viscosity of the lubricating oil composition according tothe present embodiment at 40° C. is 50 mm²/s or less, preferably 48mm²/s or less, and more preferably 45 mm²/s or less. By setting thekinematic viscosity at 40° C. to 50 mm²/s or less, necessary lowtemperature fluidity and sufficient fuel saving tend to be obtained.Moreover, the lower limit of the kinematic viscosity of the lubricatingoil composition according to the present embodiment at 40° C. is notparticularly limited, and it is preferably 20 mm²/s or more, morepreferably 30 mm²/s or more, and even more preferably 35 mm²/s or more.By setting the kinematic viscosity at 40° C. to 20 mm²/s or more, thepresent lubricating oil composition tends to be excellent in oil filmretentivity and evaporativity at lubrication sites.

Since the lubricating oil composition according to the presentembodiment has extreme pressure properties and wear resistance, whichare sufficient for achieving fuel saving, and is further capable ofreducing a metal-to-metal friction coefficient, it can be preferablyused as a gear oil for an automotive manual transmission, an automatictransmission or a continuously variable transmission, or for anindustrial gear system, and in particular, as a hypoid gear oil for thedriving systems of automobiles and railway vehicles.

EXAMPLES

Hereinafter, the present invention will be more specifically describedin the following examples. However, these examples are not intended tolimit the scope of the present invention.

Examples 1 to 18 and Comparative Examples 1 to 4

As shown in Table 1 and Table 2, the lubricating oil compositions ofExamples 1 to 18 and Comparative Examples 1 to 4 were prepared,respectively. With regard to the obtained lubricating oil compositions,their extreme pressure properties, wear resistance, seizure resistance,abrasion resistance, and oxidation stability were measured, and theresults are shown in Table 1 and Table 2.

Details of individual components shown in Table 1 and Table 2 are asfollows.

Base oil A-1: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 19mm²/s, 100° C. kinematic viscosity: 4.1 mm²/s, viscosity index: 126,pour point: −66° C., flash point: 220° C.]

Base oil A-2: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 30.3mm²/s, 100° C. kinematic viscosity: 5.9 mm²/s, viscosity index: 142,pour point: <−54° C., flash point: 246° C.]

Base oil A-3: poly-α-olefin [Group IV, 40° C. kinematic viscosity: 48mm²/s, 100° C. kinematic viscosity: 8.0 mm²/s, viscosity index: 139,pour point: −48° C., Hash point: 260° C.]

Base oil A-4: poly-α-olefin [Group IV, 40° C. kinematic viscosity; 396mm²/s, 100° C. kinematic viscosity: 39 mm²/s, viscosity index: 147, pourpoint: −36° C., flash point: 281° C.]

Base oil A-5: hydrorefined mineral oil [Group III, 40° C. kinematicviscosity: 33.97 mm²/s, 100° C. kinematic viscosity: 6.208 mm²/s,viscosity index: 133, sulfur content: less than 10 mass ppm, % C_(P):80.6, % C_(N): 19.4, % C_(A): 0]

Base oil B-1: dibasic acid ester [Group V, azelaic acid+2-ethylhexanol,40° C. kinematic viscosity: 10.3 mm²/s, 100° C. kinematic viscosity: 2.9mm²/s, viscosity index: 138, pour point: −72° C., flash point: 220° C.]

Organic molybdenum compound F-1: molybdenum dithiocarbamate (MoDTC)[amount in terms of the molybdenum element: 10% by mass]

Boron-containing dispersant G-1: borated succinimide [amount in terms ofthe boron element: 2.0% by mass, amount in terms of nitrogen element:2.3% by mass, weight-average molecular weight: 1000]

Non-boron dispersant H-1: succinimide [amount in terms of nitrogenelement: 2.3% by mass, weight-average molecular weight: 1000]

Performance additive C-1: an additive package comprising a phosphoruswear inhibitor, a sulfur extreme pressure agent, a metal deactivator, afriction modifier, a defoaming agent, etc. [amount in terms ofphosphorus element: 1.40% by mass, amount in terms of sulfur element:22.9% by mass]

Viscosity modifier J-1: a copolymer of an α-olefin and anα,β-ethylenically unsaturated dicarboxylic acid diester [weight-averagemolecular weight: 10000]

Viscosity modifier J-2: a copolymer of an α-olefin and anα,β-ethylenically unsaturated dicarboxylic acid diester [weight-averagemolecular weight: 7000]

Viscosity modifier J-3: an oligomer of ethylene and α-olefin[number-average molecular weight: 3700]

The amount of the organic molybdenum compound in terms of the molybdenumelement, the amount of the boron-containing dispersant in terms of theboron element, the amount of the performance additive in terms ofphosphorus element, and the amount of the performance additive in termsof sulfur element were obtained by an ICP elemental analysis method.

(1) Extreme Pressure Property Test

In accordance with ASTM D 2596, using a high speed four-ball tester, themaximum non-seizure load (LNSL) at 1800 rotation of each lubricating oilcomposition was measured. In the present test, it means that the largerthe maximum non-seizure load, the better the extreme pressure propertiesthat can be achieved.

(2) Wear Resistance Test

A four-ball test (ASTM D 4172) was carried out under the below-mentionedconditions, and a wear scar diameter (mm) was then measured, so thatwear resistance was evaluated. In the present test, it means that thesmaller the wear scar diameter, the better the wear resistance that canbe achieved.

Load: 800 N

Number of rotations: 1800 rpm

Temperature: 80° C.

Testing time: 30 minutes

(3) Seizure Resistance Test

Using a Falex Tester described in ASTM D 3233, a seizure load wasmeasured, and seizure resistance was evaluated. This seizure resistanceindicates an extreme pressure property between steels. Conditions forthe test are described below. In the present test, it means that thelarger the seizure load, the better the seizure resistance that can beachieved.

Temperature: 110° C.

Number of rotations: 290 rpm

(4) Abrasion Resistance Test

Using a Block on Ring Tester (LFW-1) described in ASTM D 2174, afriction coefficient was measured under the below-mentioned testconditions. Moreover, in the present test, such a friction coefficientwas obtained from both a new oil of the lubricating oil composition anda degraded oil thereof, which had been obtained by subjecting thelubricating oil to the after-mentioned oxidation stability test at 135°C. for 48 hours. In the present test, it means that the smaller thefriction coefficient, the better the abrasion resistance that can beachieved.

Ring: Falex S-10 Test Ring (SAE4620 Steel)

Block: Falex H-60 Test Block (SAE01 Steel)

Oil temperature: 90° C.

Load: 222-3113 N

Slip velocity: 0.5 m/s

(5) Oxidation Stability Test

In accordance with JIS K 2514 4. (a method for testing the oxidationstability of lubricating oil for internal combustion engines), the testwas carried out under the below-mentioned conditions, and an acid valueincrease was measured. In the present test, it means that the smallerthe acid value increase, the better the oxidation stability that can beachieved.

Temperature: 135° C.

Testing time: 96 hours

TABLE 1 Example Example Example Example Example Example 1 2 3 4 5 6 Baseoil A-1 % by mass — 40   32   — — — Base oil A-2 % by mass 80   59  63   — — — Base oil A-3 % by mass — — — 40   — — Base oil A-4 % by mass— — — 10   30   — Base oil A-5 % by mass — — — — — 80   Base oil B-1 %by mass 20   1   5   50   70   20   Base oil (mixture) kinematic  (40°C.) mm²/s 23.72 24.66 24.41 23.45 22.65 25.83 viscosity (100° C.) mm²/s 5.03  5.03  5.02  5.05  5.05  5.22 Base oil (mixture) viscosity index144    134    136    149    158    138    Additive composition (based ontotal amount of composition) Organic molybdenum compound F-1 % by mass0.5 0.5 0.5 0.5 0.5 0.5 (amount in terms of the mass ppm (500)   (500)    (500)    (500)    (500)    (500)    molybdenum element)Boron-containing dispersant G-1 % by mass 1.0 1.0 1.0 1.0 1.0 1.0(amount in terms of boron element) mass ppm (200)    (200)    (200)   (200)    (200)    (200)    Non-boron dispersant H-1 % by mass — — — — —— Performance additive C-1 % by mass 9.0 9.0 9.0 9.0 9.0 9.0 Viscositymodifier J-1 % by mass 3.5 3.5 3.5 3.5 3.5 3.5 Viseosby modifier J-2 %by mass — — — — — — Viscosity modifier J-3 % by mass 4.7 4.7 4.7 4.7 4.74.7 Kinematic viscosity  (40° C.) mm²/s 40.0  41.1  40.9  39.1  38.9 43.1  High speed four-ball test 1800 rpm LNSL N 981    981    981   1236    1236    981    Wear scar diameter mm  0.93  0.96  0.95  0.85 0.80  0.97 Falex seizure test N 4893    4671    4893    6895    7918   4715    IFW-1 test (friction coefficient) (New oil) 90° C.  0.089  0.098 0.094  0.087  0.088  0.091 (Degraded oil) 90° C.  0.030  0.045  0.040 0.031  0.035  0.033 Oxidation stability (ISOT) (Acid value mgKOH/g 2.44  2.04  2.11  2.72  2.89  2.81 increase) Example Example ExampleExample Example Example 7 8 9 10 11 12 Base oil A-1 % by mass — — — — —— Base oil A-2 % by mass 80   80   80   80  80  80  Base oil A-3 % bymass — — — — — — Base oil A-4 % by mass — — — — — — Base oil A-5 % bymass — — — — — — Base oil B-1 % by mass 20   20   20   20  20  20  Baseoil (mixture) kinematic  (40° C.) mm²/s 23.72 23.72 23.72 23.72 23.7223.72 viscosity (100° C.) mm²/s  5.03  5.03  5.03  5.03  5.03  5.03 Baseoil (mixture) viscosity index 144    144    144    144    144    144   Additive composition (based on total amount of composition) Organicmolybdenum compound F-1 % by mass 0.1 0.2 0.7 1.0 0.5 0.5 (amount interms of the mass ppm (100)    (200)    (700)    (1000)    (500)   (500)    molybdenum element) Boron-containing dispersant G-1 % by mass1.0 1.0 1.0 1.0 0.5 2.5 (amount in terms of boron element) mass ppm(200)    (200)    (200)    (200)    (100)    (500)    Non-borondispersant H-1 % by mass — — — — — — Performance additive C-1 % by mass9.0 9.0 9.0 9.0 9.0 9.0 Viscosity modifier J-1 % by mass 3.5 3.5 3.5 3.53.5 3.5 Viseosby modifier J-2 % by mass — — — — — — Viscosity modifierJ-3 % by mass 4.7 4.7 4.7 4.7 4.7 4.7 Kinematic viscosity  (40° C.)mm²/s 40.0  40.0  40.0  40.0  39.9  41.0  High speed four-ball test 1800rpm LNSL N 981    981    981    1236    981    1236    Wear scardiameter mm  0.97  0.95  0.92  0.94  0.97  0.85 Falex seizure test N5382    5249    4671    4359    4537    6672    IFW-1 test (frictioncoefficient) (New oil) 90° C.  0.091  0.090  0.090  0.091  0.087  0.093(Degraded oil) 90° C.  0.046  0.041  0.031  0.036  0.030  0.043Oxidation stability (ISOT) (Acid value mgKOH/g  2.41  2.41  2.51  2.64 2.45  2.45 increase) Example Example Example Example Example Example 1314 15 16 17 18 Base oil A-1 % by mass — — — — — — Base oil A-2 % by mass80  80  80  80  80  80  Base oil A-3 % by mass — — — — — — Base oil A-4% by mass — — — — — — Base oil A-5 % by mass — — — — — — Base oil B-1 %by mass 20  20  20  20  20  20  Base oil (mixture) kinematic  (40° C.)mm²/s 23.72 23.72 23.72 23.72 23.72 23.72 viscosity (100° C.) mm²/s 5.03  5.03  5.03  5.03  5.03  5.03 Base oil (mixture) viscosity index144    144    144    144    144    144    Additive composition (based ontotal amount of composition) Organic molybdenum compound F-1 % by mass0.5 0.5 0.5 0.5 0.5 0.5 (amount in terms of the mass ppm (500)   (500)    (500)    (500)    (500)    (500)    molybdenum element)Boron-containing dispersant G-1 % by mass 1.0 1.0 1.0 1.0 1.0 1.0(amount in terms of boron element) mass ppm (200)    (200)    (200)   (200)    (200)    (200)    Non-boron dispersant H-1 % by mass — — — — —— Performance additive C-1 % by mass 9.0 9.0 9.0 9.0 9.0 9.0 Viscositymodifier J-1 % by mass 2.5 7.0 10.0  15.0  5.0 — Viseosby modifier J-2 %by mass — — — — 17.0  22.0  Viscosity modifier J-3 % by mass 5.3 3.0 1.0— — — Kinematic viscosity  (40° C.) mm²/s 39.8  39.4  38.7  39.9  40.8 40.8  High speed four-ball test 1800 rpm LNSL N 981    981    981   981    981    981    Wear scar diameter mm  0.90  0.95  0.96  0.94  0.92 0.91 Falex seizure test N 4893    4493    4404    4359    5160   5382    IFW-1 test (friction coefficient) (New oil) 90° C.  0.090  0.096 0.095  0.093  0.094  0.095 (Degraded oil) 90° C.  0.031  0.038  0.042 0.033  0.035  0.031 Oxidation stability (ISOT) (Acid value mgKOH/g 2.42  2.43  2.45  2.48  2.47  2.45 increase)

TABLE 2 Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 4 Base oil A-1 % by mass 42 — — — Base oilA-2 % by mass 58 — 80 80 Base oil A-3 % by mass — — — — Base oil A-4 %by mass — 20 — — Base oil A-5 % by mass — — — — Base oil B-1 % by mass —80 20 20 Base oil (mixture) kinematic viscosity (40° C.) mm²/s 24.7217.04 23.72 23.72 (100° C.) mm²/s 5.03 4.13 5.03 5.03 Base oil (mixture)viscosity index 134 151 144 144 Additive composition (based on totalamount of composition) Organic molybdenum compound F-1 % by mass 0.5 0.5— 1.2 (Amount in terms of the molybdenum mass ppm (500) (500) — (1200)element) Boron-containing dispersant G-1 % by mass 1.0 1.0 1.0 1.0(Amount in terms of boron element) mass ppm (200) (200) (200) (200) Non-boron dispersant H-1 % by mass — — — — Performance additive C-1 % bymass 9.0 9.0 9.0 9.0 Viscosity modifier J-1 % by mass 3.5 3.5 3.5 3.5Viscosity modifier J-2 % by mass — — — — Viscosity modifier J-3 % bymass 4.7 4.7 4.7 4.7 Kinematic viscosity (40° C.) mm²/s 41.2 33.3 40.040.1 High speed four-ball test 1800 rpm LNSL N 785 1236 981 1236 Wearscar diameter mm 1.23 0.95 1.11 0.96 Falex seizure test N 3870 8363 53384003 IFW-1 test (friction coefficient) (New oil) 90° C. 0.098 0.0880.090 0.094 (Degraded oil) 90° C. 0.095 0.036 0.091 0.048 Oxidationstability (ISOT) (Acid value increase) mgKOH/g 2.1 3.11 2.23 2.83

As is apparent from Table 1 and Table 2, it was found that thelubricating oil compositions of Examples 1 to 18 were better in extremepressure properties, wear resistance, seizure resistance, abrasionresistance and oxidation stability, and had a smaller metal-to-metalfriction coefficient, than the lubricating oil compositions ofComparative Examples 1 to 4.

1-4. (canceled)
 5. A lubricating oil composition comprising: alubricating base oil comprising 1% to 70% by mass of an ester base oilbased on the total amount of the lubricating base oil, and saidlubricating base oil having a kinematic viscosity at 40° C. of 22 to 26mm²/s; wherein the amount of the lubricating base oil is 67.5% by massor more; and molybdenum dithiocarbamate in an amount of 100 to 1000 massppm in terms of molybdenum element based on the total amount of thelubricating oil composition, wherein the lubricating oil composition hasa kinematic viscosity at 40° C. of 10 of 50 mm²/s or less, thelubricating oil composition further comprising 2.5% to 24% by mass of acopolymer of an α-olefin and an α,β-ethylenically unsaturateddicarboxylic acid diester, based on the total amount of the lubricatingoil composition, wherein the weight-average molecular weight of thecopolymer is 6000 to 15000, the lubricating oil further comprising aboron-containing dispersant in an amount of 100 to 500 mass ppm in termsof boron element based on the total amount of the lubricating oilcomposition, wherein the lubricating oil composition is free ofstyrene-maleic anhydride ester.
 6. The lubricating oil compositionaccording to claim 5, wherein the lubricating base oil comprises asynthetic base oil comprising a poly-α-olefin.
 7. A method oflubricating a hypoid gear comprising utilizing the lubricating oilcomposition of claim 5.